Abstract: The combination of a recently proposed linear interpolation method (LIM)
[Senjean et al., Phys. Rev. A 92, 012518 (2015)], which enables the calculation
of weight-independent excitation energies in range-separated ensemble
density-functional approximations, with the extrapolation scheme of Savin [J.
Chem. Phys. 140, 18A509 (2014)] is presented in this work. It is shown that LIM
excitation energies vary quadratically with the inverse of the range-separation
parameter mu when the latter is large. As a result, the extrapolation scheme,
which is usually applied to long-range interacting energies, can be adapted
straightforwardly to LIM. This extrapolated LIM (ELIM) has been tested on a
small test set consisting of He, Be, H2 and HeH+. Relatively accurate results
have been obtained for the first singlet excitation energies with the typical
mu=0.4 value. The improvement of LIM after extrapolation is remarkable, in
particular for the doubly-excited 2^1Sigma+g state in the stretched H2
molecule. Three-state ensemble calculations in H2 also show that ELIM does not
necessarily improves relative excitation energies, even though individual
excitation energies are more accurate after extrapolation. Finally, an
alternative decomposition of the short-range ensemble exchange-correlation
energy is proposed in order to correct for ghost-interaction errors in
multideterminant range-separated ensemble density-functional theory
calculations. The implementation and calibration of such a scheme is currently
in progress.